[hotfix] fix zero comm buffer init (#6154)

Author: ver217

colossalai/zero/low_level/low_level_optim.py

@@ -371,7 +371,7 @@ def _run_reduction(self):
                     for i, sz in enumerate(bucket_store.sizes):
                         grp = bucket_store.torch_pg if len(bucket_store.sizes) == 1 else bucket_store.torch_pg[i]
                         flat_grads_list = list(cur_flat_grads.split(len(cur_flat_grads) // sz))
-                        received_grad = torch.zeros_like(flat_grads_list[0])
+                        received_grad = torch.empty_like(flat_grads_list[0])
                         if self._fp8_communication:
                             reduce_scatter_fp8(
                                 received_grad,